The use of radiation therapy for the treatment of cancer is well known. Typically, radiation therapy involves focusing a beam of radiation (e.g., proton, x-ray, or electron radiation) onto a target volume to diagnose an afflicted area or to monitor a tumor or lesion. A beam of high energy proton, x-ray, or electron radiation (“therapeutic radiation”) is subsequently directed into the monitored area to treat the area. During treatment, the area continues to be monitored to ensure appropriate positioning of the therapeutic radiation beam.
A radiation therapy device typically includes a surface (e.g., a couch) to support the patient and an overhead radiation source that emits the therapeutic radiation beam. The radiation source directs the beam into the targeted volume (e.g., the tumor being treated) in the patient, who is positioned directly below the radiation source while in a supine position on the couch. The radiation beam may be moved to “paint” the target volume, or the radiation beam may be shaped so that its cross-section approximates the shape of the target volume, so that the beam falls only on the target volume and not on surrounding, healthy tissue. The strength or intensity of the beam is selected depending on the thickness of the target volume and its depth within the patient, as well as other factors.
During treatment, the patient may move. For example, the patient may shift and/or rotate his or her body during treatment, changing both the focal point of the incident radiation beam and the angle of the beam relative to the targeted volume. As a result, the beam may no longer be pointed at just the target volume, and/or the cross-section of the beam at the point where it intersects the target volume may change so that it no longer approximates the shape of the target volume. Consequently, the radiation beam may not cover the entire target volume or might land on tissue outside the target volume.
The target volume itself may move during treatment even if the patient does not. For example, the patient's breathing may cause the target volume to move up and down and hence closer to and then further away from the radiation source. When the target volume is closer to the radiation source, the radiation beam might not cover the entire target volume, and when the target volume is further away from the radiation source, the beam might also land on tissue outside the target volume. Also, the intensity of the beam at the target volume may alternately increase and decrease, and consequently the target volume might not receive a uniform dose of radiation or might not receive the total dose prescribed in the treatment plan.